(1) Field of Invention
The present invention relates to a heart valve system and, more particularly, to a percutaneous mitral valve for delivery and implantation at a mitral position.
(2) Description of Related Art
Valvular heart disease is the third most common cause of cardiovascular disease in the United States. Mitral Regurgitation (MR) is a common valvular disorder, which can be manifested, in acute and chronic forms. Both the acute and chronic forms of MR are the source of a significant amount of cardiovascular morbidity and mortality. Dysfunction in the mitral valve can arise from abnormalities of any part of the mitral valve apparatus, including the leaflets, annulus, chordae tendineae, and papillary muscles. Additional anatomical support for mitral valve function comes from the loft atrial wall and ventricular myocardium adjacent to the papillary muscles. Proper valve function depends on the interaction of all of the anatomic components and a minor dyssynchrony can result in significant valvular dysfunction. With the deranged valvular structure and/or function permitting back/low there is a resultant left ventricular volume overload. Over time and with deterioration of the mitral valve function, this volume overload results in left ventricular dilation and dysfunction. Left ventricular dysfunction in conjunction with MR can lead to pulmonary hypertension, congestive heart failure and ultimately death. Each year in the United States, there are more than 500,000 patients discharged with the diagnosis of MR, and annually in the United States, some 18,000 patients undergo mitral valve surgery. These statistics illustrate the gravity of this problem and the immense cost burden that it creates.
When addressing MR and its etiologies, it must first be identified if the pathologic regurgitation is a result of a primary abnormality of the valve apparatus or secondary to another cardiac disease. When MR is due to a primary abnormality of the valve apparatus, it is referred to as primary MR. The most common causes of primary MR are mitral valve prolapse, rheumatic heart disease and infective endocarditis. Far less common causes of primary MR include trauma and congenital heart disease such as a valve cleft. Secondary MR is most commonly due to ischemic heart disease, left ventricular systolic dysfunction and dilatation (i.e., Functional MR) and least commonly hypertrophic cardiomyopathy. Finally, in the elderly, annular calcification is a cause of MR, however this rarely progresses past moderate and infrequently requires intervention.
Correction of MR within a certain window minimizes the consequences described above. There is a scientifically well-established cause-and-effect relationship between pathologic MR and its deleterious effects on the left ventricle and the patient's life, in the absence of a secondary cause, it is the abnormal valve that makes the heart and thus the patient sick. Definitive therapeutic options for severe MR remain few and the only truly corrective therapies, which require surgical intervention—commonly associated with a median sternotomy—are presently effective. The currently practiced techniques consist of mitral valve repair and replacement. MR is a mechanical problem, thus medical therapy has been shown to be inadequate, and a mechanical intervention (e.g., repair or replacement) is required to improve mortality. Valve competence needs to be restored in order to remove the volume overload and its deleterious consequences. Another controversy within the field of mitral valve repair and replacement is the timing of the intervention.
Currently, decisions are based on as host of factors including symptoms, quantification of left ventricular ejection fraction, age, functional capacity, regurgitant fraction, regurgitant orifice area and regurgitant volume. Imaging and calculation of quantitative measures are performed primarily by Echocardiography. These factors can be subjective and inaccurate. This leads to eligible patients being passed over and perhaps some patients having operations unnecessarily. Finally, a substantial subset of patients is deemed to not be surgical candidates, due to either co-morbid medical illness, age or other factors.
Percutaneous replacement of a heart valve is an incredible development in patient care and one of the great recent breakthroughs in cardiovascular medicine. However, it has been difficult to apply this technology to the mitral valve given its unique anatomical position close to the left ventricular outflow tract. Thus, development of a percutaneous system for mitral valve replacement has not yet been effectively achieved. Development of a percutaneous technology, which has been proven possible in the aortic position would allow for a cure to a very prevalent human disease, while also alleviating a significant amount of suffering associated with both the disease and the current therapeutic options, and finally allowing a more broad range of patients to benefit from the minimally invasive intervention.
The percutaneous approach to valve replacement is a welcome option for many patients due to its sparing of aggressive surgery and reducing the associated comorbidities based on the minimally invasive nature of the procedure. The lure of percutaneous technologies lies in providing cost-effective solutions to heart valve disease, thereby allowing more timely interventions with acceptable efficacy and minimal complications, especially for patients who cannot undergo surgery. These technologies can help avoid open heart surgery in severely ill patients and reduce the number of reoperations in young patients with congenital heart defects.
Nevertheless, there exists numerous challenges in the design and fabrication of a percutanously delivered mitral valve. For example, one challenge is the development of a system that will secure the valve in place and developing a fully functional and durable valve that can be crimped into a catheter. Transcatheter aortic valve implantation takes advantage of the fact that the stenotic aortic valve is heavily calcified. Thus a stented design is ideal as the calcium acts as an anchor for the stem and keeps the valve from migrating. Placing a stented valve in a non-calcified aortic valve would create a much higher risk for valve embolization. The predominant disease process of the mitral valve is mitral regurgitation. This disease is not generally associated with a heavily calcified valve, although that can be the case. Therefore, a fixation apparatus of a percutaneous mitral valve is critical to maintain valve position in the face of physiologic stress. The developed valve should also be robust enough to last as long as commercially available bin-prosthetic valves yet have a low enough profile that can be delivered though a catheter. Again this challenge is one that can be overcome with careful design and utilizing the natural design that evolution has given to the native mitral valve.
With respect to percutaneous delivery, there are not any currently ongoing clinical trials evaluating a percutaneous valve delivery system for the mitral valve diseases. The mitral valve position presents unique challenges for the placement of a transcatheter valve, including, but not limited to inherent anatomic features of the mitral valve (MV) that make fixation and perivalvular seal with currently available devices a challenge, the lack of a calcium bed to fix the valve, and challenges in delivery catheter size due to the increased annulus diameter of the mitral when compared to the aortic valve. Additionally, there is the question as to the configuration of the prosthetic leaflets, as there may be a potential physiologic advantage of the asymmetric vortex bubble and elliptical flow profile that forms through a bi-leaflet valve, compared to the symmetric, round vortex bubble that develops through as tri-leaflet valve. Finally, there is general consensus that the saddle-shape annulus of the mitral valve is a critical component of the left heart complex, it serves a major role in left ventricular function by helping to maintain LV shape, creating efficient valve closure, robust ventricular filling, and chamber contractility. Destruction of the mitral valve apparatus at the tune of mitral valve replacement causes an immediate decrease in chamber contractility and an increase in afterload as the radius term in the Laplace equation increases. It is therefore crucial to maintain some semblance of proper annulus morphology when creating a percutaneous mitral valve, which does not apply to the aortic valve, and thus again illustrates the importance of this valve and delivery system.
Thus, a continuing need exists for a well-designed percutaneous technology for mitral valve replacement that would revolutionize the treatment of valvular heart disease for millions of people.